1. Field of the Invention
The present invention relates generally to transducers for inducing vibrational signals in an elastic medium, and more particularly, but not by way of limitation, it relates to an improved construction for a transducer especially suitable for generating seismic shear waves in the earth for the purpose of seismographic surveying.
2. Description of the Prior Art
When practicing the system of vibrational seismographic exploration disclosed generally in U.S. Pat. No. 2,688,124 issued to Doty et al. on Aug. 31, 1954, a seismic signal of controlled frequency must be induced in the earth. The controlled signal then propagates downwardly and is reflected from subterranean interfaces upwardly and recorded as a composite signal having a plurality of the reflected controlled signals at various time phase positions on the record. The composite record is then correlated with the original transmitted signal to produce highly valuable seismic data.
A prior art transducer of general type is shown in U.S. Pat. No. 3,159,233 issued to Clynch et al. on Dec. 1, 1964. In general, such a transducer is comprised of a mass member having a cylinder bore extending therethrough, a piston member having a piston and oppositely extending rods reciprocally disposed in the cylinder bore, and means for introducing fluid under pressure to the cylinder bore causing reciprocation of the piston member relative to the mass member. These specific types of prior transducers had a particular problem that was inherent in the design; the problem was primarily concerned with the tendency of the mass member to rotate about the piston.
Prior art attempts at plural vibrator assemblies is particularly characterized by U.S. Pat. No. 3,777,843 as issued Dec. 11, 1973 in the name of Fair et al. and entitled "Seismic Vibrator System." This system utilized two separate vibrators earth-coupled through a single, unitary base plate in an attempt to better distribute vibrational forces over the whole of the base plate; however, systems such as this proved to be difficult to synchronize and subsequently were relegated to limited usage.